In prior art, low dropout voltage regulators are commonly used to supply power for circuits. Therefore, in a low dropout voltage regulator, the transistor for outputting power has to endure great current loading, and has to be implemented with a great area. In addition, since the circuit may be switched between different operation modes, the output voltage and output current of the low dropout voltage regulators will also change. If the variations of the voltage and current are rather severe and exceed the safe operating area (SOA) of the transistor in the low dropout voltage regulator, then the transistor would break down, causing abnormal behavior of the low dropout voltage regulator and even damaging the low dropout voltage regulator.
For example, in the wireless communication application, the low dropout voltage regulator can provide power for the power amplifier. When the power amplifier is to be changed from a high power mode to a low power mode, the low dropout voltage regulator can lower its output voltage so the power of the power amplifier can be lowered accordingly. However, in this case, the cross voltage endured by the transistor in the low dropout voltage regulator will increase, and may exceed the SOA of the transistor easily, causing instability to the system.